The invention relates generally to vertical cavity semiconductor lasers. More particularly, the invention relates to a method for controlling and stabilizing the polarization mode of vertical cavity surface emitting lasers.
Vertical cavity surface emitting lasers (VCSELs) are key components in many optical communication systems. Communication systems based on wavelength division multiplexing (WDM) require a wide range of VCSEL operating wavelengths. This capability is realized, for example, by employing a micro-electro-mechanical (MEM) structure to adjust, or tune, the length of the VCSEL cavity.
The VCSEL cavity generally supports two polarization modes due to the circular symmetry of the cavity and the isotropic properties of the semiconductor materials. In practice, however, a preferential excitation of one of the polarization modes occurs due to imperfections in the circular symmetry and material isotropy. Unfortunately, the resulting polarization mode often exhibits instability. A polarization xe2x80x9cflipxe2x80x9d from one polarization mode to the other polarization mode can be induced by a minor perturbation in environmental and operational conditions. For example, fluctuations in the ambient temperature or a change in the driving current or in the optical pump power can cause the polarization of the VCSEL beam to switch between polarization modes.
The effect of externally applied stress on the polarization mode of VCSELs has been demonstrated to affect polarization stability. Such lasers require significant hardware to apply the stress to an as-built unit and are not practical for large scale production of VCSEL devices. What is needed is a robust scheme to control the polarization state of VCSELs over a wide range of operation wavelengths that does not require a post-processing adjustment of the VCSEL.
In one aspect, the invention features a semiconductor laser for generating an optical beam having a stable polarization mode. The semiconductor laser includes a semiconductor structure having an active layer and a surface. The semiconductor laser also includes a stressor adapted to apply a stress to the semiconductor structure to generate a gain anisotropy and a birefringence in the semiconductor structure to stabilize the polarization of the optical beam generated by the semiconductor. In one embodiment, the active layer includes at least one quantum well disposed at a distance from the surface of the semiconductor structure to generate a predetermined gain anisotropy in response to the applied stress. In another embodiment, the active layer includes at least one quantum well having a first region including a first material, a second region including a second material, and an interface between the two regions. A plurality of atomic bonds of different lengths is established across the interface to produce a gain anisotropy in the active layer.
In another aspect, the invention features a semiconductor laser for generating an optical beam having a stable polarization mode. The semiconductor laser includes a semiconductor structure having an active layer. The active layer includes a first semiconductor layer having a first material. The active layer also includes a second semiconductor layer adjacent to the first semiconductor layer and having a second material. A plurality of atomic bonds of different lengths are established across the interface to produce a gain anisotropy in the active layer. In one embodiment, the semiconductor laser also includes a stressor adapted to apply a stress to the semiconductor structure. The stressor induces a birefringence in the semiconductor structure responsive to the applied stress. The polarization mode of the optical beam is stablilzed in response to the gain anisotropy and the birefringence. In another embodiment, the active layer is disposed at a distance from the stressor to generate a predetermined gain anisotropy in response to the applied stress.
In another aspect, the invention features a method for fabricating a semiconductor laser having a stable polarization mode. The method includes forming a semiconductor structure having a quantum well at a predetermined distance from a surface of the semiconductor structure. The method also includes forming a stressor on the surface of the semiconductor structure. The stressor induces a gain anisotropy in the quantum well in response to the predetermined distance and the stressor induces a birefrinence in the semiconductor structure.